People say Einstein suggested that 98% of the world’s population couldn’t figure out the answer to this puzzle. It is not for the faint of heart:

There are 5 houses each with a different color. Their owners, each with a unique heritage, drinks a certain type of beverage, smokes a certain brand of cigarette, and keeps a certain variety of pet. None of the owners have the same variety of pet, smoke the same brand of cigarette or drink the same beverage.

Clues:

The Brit lives in the red house.

The Swede keeps dogs as pets.

The Dane drinks tea.

Looking from in front, the green house is just to the left of the white house.

The green house’s owner drinks coffee.

The person who smokes Pall Malls raises birds.

The owner of the yellow house smokes Dunhill.

The man living in the center house drinks milk.

The Norwegian lives in the leftmost house.

The man who smokes Blends lives next to the one who keeps cats.

The man who keeps a horse lives next to the man who smokes Dunhill.

The owner who smokes Bluemasters also drinks beer.

The German smokes Prince.

The Norwegian lives next to the blue house.

The man who smokes Blends has a neighbor who drinks water.

(Note: I found this puzzle and its solution at
http://www.brain-fun.com/Brain-Teasers/EinsteinsRiddle.php)

HOUSES: The Norwegian lives in the leftmost house, next to blue house. The owner of the center house drinks milk, therefore, is not a green house (whose owner drinks coffee). The green house is #4 since it is to the left of the white house and there is no other place for a pair of houses to fit. Therefore, the center house must be red and, therefore, the Brit’s house. The Norwegian’s house is, therefore, yellow. The Dane drinks tea and doesn’t live in the coffee-drinking green house. The Swede keeps dogs so he is not the person who owns horses and lives in the blue house.

DRINKS: The man in the center house, the Brit, drinks milk. The green house’s owner drinks coffee. The Dane drinks tea so the Dane does not live in the green house. The one who smokes Bluemasters drinks beer. And since the German smokes Prince we know that the German does not drink beer. The Brit drinks milk and so does not smoke Bluemasters. Since the Norwegian smokes Dunhills, he can’t do the Bluemasters/Beer duo. And the Dane drinks tea, so the Swede must drink the beer.

SMOKES: The owner of the yellow house, the Norwegian, smokes Dunhill. The German smokes Prince. The man who smokes Blends has a neighbor who drinks water, and the green house #4 drinks coffee. Therefore, the person in the white house #5 doesn’t smoke Blends. The Dane does not smoke Bluemasters since he drinks tea and not beer. The Brit does not smoke Bluemasters since he drinks milk. The Norwegian doesn’t smoke Bluemasters since he smokes Dunhills. And the German smokes Prince, not Bluemasters. So that means the Swede must smoke Bluemasters. Therefore, the Swede also drinks beer.

PETS: The man who keeps a horse lives next door to the Dunhill smoker, the Norwegian, so the owner of the blue house owns a horse. Since the Norwegian smokes Dunhills, they do not have birds. The Swede keeps dogs, so he is not the man who owns horses and lives in the blue house. Whomever smokes the Bluemasters doesn’t own birds.

When we find out the Swede smokes the Bluemasters and drinks beer, and also know he keeps dogs, we know everything about him except which house he lives in. He doesn’t live in the red house because he drinks beer not milk. He doesn’t live in the green house because he drinks beer not coffee. He is not in the yellow house smoking Dunhills and since he doesn’t own a horse, he isn’t in the blue house. Therefore, the Swede is in the white house.

The blue house and the green house are still unclaimed. The Dane must be in the Blue house drinking tea, because he doesn’t drink coffee, therefore, can’t be in the green house. We already know that the Brit is in the red house and the Norwegian is in the yellow house. So finally, we can put the German into the green house.

So now we have the German smoking Princes and drinking coffee in the green house. The Swede is in the white house smoking Bluemasters and drinking beer with his dogs. The Norwegian is in the yellow house smoking Dunhills. The Dane is in the blue house drinking tea and owning a horse. Since he has a horse, he can’t have birds and, therefore, doesn’t smoke Pall Malls, but rather Blends. The Brit is in the red house drinking milk.

Since the Dane in the blue house smokes Blends, his neighbor must drink water. The house next door, the Brit, already drinks milk, so the Norwegian on the other side must be the one who drinks water. We know what everyone smokes except the Brit, who by default smokes Pall Malls, and then also owns birds. Since he owns the birds, the neighbor of the Blends smoker who owns cats is the Norwegian, leaving only one solution: The German owns the fish.